Overview:

Brachymyrmex obscurior is a small, dull brown, non-descript and soft-bodied ant that is associated with marginal and synanthropic habitats. Wilson and Taylor (1967) raised B. obscurior to species status, qualifying their decision as a "purely provisional measure, contingent upon a fuller revision of the large and difficult genus it which it belongs." It is native to the Neotropics and introduced to regions including the Pacific Islands, and the Netherlands.

Distribution Notes:

Biology:

The species is known to nest in the ground (Tschinkel & Hess, 1999), is especially well adapted to extreme, marginal environments (Morrison, 2006), and can be a common visitor to extrafloral nectaries (Koptur et al., 2010; Piovia-Scott, 2011). Brachymyrmex obscurior is considered a minor urban pest in Florida (Klotz et al., 1995), where it is rare but widespread (Deyrup, 2003a). Brachymyrmex obscurior is native to the Neotropics, and is ubiquitous across the Caribbean. The species is so common among those islands, that it is often the only single ant species to occur on the smallest cays (Morrison, 2006). However, the absence of B. obscurior from larger islands with more diverse ant faunas suggests that the species may be a poor competitor against most other ants (Morrison, 2006). The introduced range of B. obscurior includes Hawaii (Krushelnycky et al., 2005), Samoa (Wilson & Taylor, 1967), and the Solomon Islands (E.M. Sarnat, pers. obs.) in the Pacific, the southern United States (Dash et al., 2005; Deyrup, 2003a; Ipser et al., 2005), and the Netherlands (Boer & Vierbergen, 2008). Records from Antweb also indicate the species occurs in greenhouses and butterfly houses in the state of Washington.

In Costa Rica (Jack Longino)This is a widespread species, common in other parts of the world in synanthropic habitats. In Costa Rica it is less common than the related B. heeri, a similarly synanthropic species. I know B. obscurior from two collections in Costa Rica, both coastal. One collection is from a beach site 25km North of Cahuita on the Atlantic coast, the other is from the town square of Sierpe, a small port town in a mangrove estuary on the Pacific coast.

Identification:

Diagnosis among workers of introduced and commonly intercepted species. Antenna 9-segmented. Antennal club indistinct. Antennal scapes surpassing the posterior margin of head by more than 1/5th their length. Eyes of moderate size (greater than 5 facets). Eye length less than malar distance. Head with or without distinct ocelli. Antennal sockets and posterior clypeal margin separated by a distance less than the minimum width of antennal scape. Dorsum of mesosoma lacking a deep and broad concavity. Metanotal groove present. Pronotum and mesonotum with pairs of erect hairs. Propodeum and petiolar node both lacking a pair of short teeth. Propodeum lacking posteriorly projecting protrusion. Metapleuron with a distinct gland orifice. Waist 1-segmented (may be hidden by gaster). Petiolar node appearing flattened. Gaster armed with acidopore. Gaster (especially first segment) with dense pilosity, giving it a dull appearance. Color dull brown.

Brachymyrmex obscurior can be distinguished from most other introduced members of the genus by the following combination of characters: (1) dense pubescence on the first gastral tergite, (2) antennal scapes exceeding posterior margin of head by at least 1/5 their length, (3) erect hairs on the pronotum and mesonotum, (4) eye length distinctly less than malar length, and (5) dull brown in color. MacGown (2012) offers that the species, is most similar to B. patagonicus, but can be differentiated by the much denser pubescence, especially on the gaster. Males of B. obscurior are concolorous dark brown; whereas males of B. patagonicus are bicolored with the head and gaster dark brown and the rest of the body, including the appendages, being pale yellowish-brown. Additional characters cited by MacGown (2007) to separate the two include the smaller eye of B. obscurior which is shorter than the malar space, compared to the larger eye of B. patagonicus which is approximately equal to the malar length. 

Specimen Habitat Summary

Found most commonly in these habitats: 1 times found in beach edge, 1 times found in pine/oak/thorn scrub, 1 times found in thorn scrub, 1 times found in tropical dry scrub, 1 times found in dist. habitat, 1 times found in Greenhouse, 1 times found in pine forest, 1 times found in riparian wet forest, 1 times found in 1¡ forest, 1 times found in town center, ...

Found most commonly in these microhabitats: 2 times tending Hemiargus ceraunus on "Mimosa" sp., 1 times nest under rock, 1 times foraging on tree, 1 times foragers under rock, 2 times foragers, 1 times ex sifted leaf litter, 1 times tree buttress litter, 1 times strays, 1 times sifted litter (leaf mold, rotten wood), 1 times efn Passiflora incarnata, 1 times butterfly house of Pacific Science Center, ...

Collected most commonly using these methods: 11 times search, 7 times Ground forager, 1 times on spam bait, 2 times hand collecting, 1 times maxiWinkler, 1 times DaJuns leaf litter sifting, 1 times Foraging on tree, 1 times litter sample, 1 times DaJuns litter sifting, 1 times Ex rotten branch on ground, sweeping times ex, ...

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